Bisphenol-A is prepared commercially by reaction of phenol and acetone in contact with an acidic catalyst such as hydrochloric acid or an acidic ion exchange resin such as a sulfonic acid substituted polystyrene. As would be expected, the reaction is not 100% directed to the formation of bisphenol-A. A significant number and quantity of undesirable by-products, some of which are highly colored are also formed. These undesirable by-products affect bisphenol-A yield and quality. They are particularly significant when bisphenol-A is converted into polycarbonate. A high degree of purity of bisphenol-A with respect to wt. % impurities and color of the bisphenol-A is therefore required for preparation of quality polycarbonate.
In preparing the bisphenol-A, the acetone is contacted with a substantial excess of phenol, usually 6 times or more the stoichiometric amount necessary while in the presence of the acidic catalyst. The initial separation of bisphenol-A from excess phenol and by-products from the reactor effluent is usually carried out by distillation separation of phenol and bisphenol or phenol bisphenol-A adduct crystallization.
In this manner, the major portion of the bisphenol-A is removed from the by-products and impurities. There are a number of methods for handling the impurities by-products rich stream after removal of the major portion of the bisphenol-A. For example, such a stream can be (a) partially recycled to the condensation reactor, (b) fed to an "isomerization" reactor to convert various bisphenol-A geometric isomers and related compounds to bisphenol-A, (c) excess phenol removed, (d) burned, (e) distill off further bisphenol-A and the like. Generally when bisphenol-A is recovered from these streams the recovery method is distillation.
We have discovered a new, feasible and economic system to recover bisphenol-A from streams, usually referred to as purge streams, which remain after the first major portion of bisphenol-A has been removed, usually by distillation or phenol/bisphenol-A adduct formation. This purge stream is comprised of bisphenol-A, zero to small or even large quantities of phenol depending on whether excess phenol is partially or completely removed prior to the first major isolation of bisphenol-A or thereafter, and the remainder of the stream comprised of bisphenol isomers and somewhat higher molecular weight and related compounds, generally referred to as tar. This new method is advantageous compared to a distillation method because it has lower investment costs, lower operating costs, lower operating temperature. The process provides a product with improved quality.